Molecular sieving silica membrane fabrication process

ABSTRACT

A process for producing a molecular sieve silica membrane comprising depositing a hybrid organic-inorganic polymer comprising at least one organic constituent and at least one inorganic constituent on a porous substrate material and removing at least a portion of the at least one organic constituent of the hybrid organic-inorganic polymer, forming a porous film.

This invention was made in the course of work authorized by U.S.Department of Energy Contract No. DE-ACD4-94AL85000. The Government hascertain rights in this invention.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part application of a U.S. patentapplication filed Aug. 23, 1996 and having Ser. No. 08/702,745, U.S.Pat. No. 5,770,275.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for fabrication of molecular sievingsilica membranes suitable for use in gas separation applications.

2. Description of Prior Art

A gas separation membrane can be described as a semi-permeable organicor inorganic barrier capable of separating gases by virtue ofdifferences in diffusion coefficients, solubility, or size. At hightemperatures, facilitated transport mechanisms, such as selectiveadsorption or capillary condensation, are generally not operative. As aresult, membrane selectivity is mainly achieved by size exclusion.Inorganic membranes, owing to their presumed superior thermal,mechanical, and chemical stability, compared to that of organic polymermembranes, have been proposed as replacements for energy intensiveindustrial separation processes such as distillation, drying, andevaporation. In addition to consuming less energy than conventionalindustrial separation processes, membrane systems are compact andmodular, enabling easy incorporation into existing industrial processes.In order to be commercially feasible, gas separation membranes shouldexhibit high selectivity, which is achieved by having small pore sizesand narrow pore size distributions, and high permeance, which is definedas flux/pressure drop, achieved by having a large volume fractionporosity and a very thin selective layer. The selectivity, or separationfactor, of the membranes pertaining to this invention is defined as theratio of the permeance of the faster permeating gas to the permeance ofthe slower permeating gas. These membranes have potential applicationsin separations such as dehydrogenation, the separation of nitrogen frommethane in ammonia plants, the production of oxygen and nitrogen fromair, enhanced oil recovery, the separation of carbon dioxide andnitrogen from methane in natural gas processing and carbon dioxiderecovery from land fills. To date, however, the potential applicationsof these membranes have not been realized due to the difficulty inobtaining very small pore sizes, and more importantly, due to thecurrent procedures used for obtaining very small pore sizes which oftenresult in an unfavorable reduction of the gas flux through the membrane.

Membranes for gas separation can be made using organic or inorganicroutes. Organic membranes generally exhibit high separation factors forvarious gases of industrial interest, but their intrinsic permeabilityis very low. Gas transport through organic membranes occurs through asolution diffusion mechanism in which the permeation process iscontrolled by the molecular diffusion of gases in a dense organicpolymer matrix. Several studies have shown that due to some intrinsicpolymer property, such as free volume, there is an apparent trade-offbetween permeability and selectivity independent of chosen gas pair orpolymer. See, for example, Robeson, L. M., Journal of Membrane Science,62 (1991) pages 165-185. In addition to this trade-off, organicmembranes have several other potential disadvantages including limitedthermal stability, limited chemical stability, especially to organicsolvents, and poor mechanical strength.

In contrast, porous inorganic membranes overcome many of the inherentlimitations of organic membranes because there is no intrinsicrelationship between permeability and selectivity. Permeability iscontrolled by volume fraction porosity, whereas selectivity isdetermined by the pore size and pore size distribution. Size-selectivegas separation using porous inorganic membranes is by far the mostattractive way to separate gas mixtures of industrial importance fromthe standpoint of energy consumption and economics. The combination ofsmall pore sizes, narrow pore size distribution, high porosity, withtailored pore topology, pore surface chemistry, and surfaceadsorption/diffusion characteristics makes these membranes attractivefor a wide range of applications including ultrafiltration,microfiltration, or gas separation, such as dehydrogenation,nitrogen/methane separation in ammonia plants, oxygen/nitrogenseparation from air, enhanced oil recovery, separation of carbon dioxideand nitrogen from methane in natural gas processing, and carbon dioxiderecovery from land fills. The increasing industrial requirements for lowcost gases with high purity has provided a strong impetus towardsdeveloping inorganic membranes with unique separative properties.Inorganic membranes are prepared from both particulate and polymericprecursors with a wide range of pore sizes and porosities.

The particulate approach to preparing inorganic membranes involvesslip-casting and calcination of a charged stabilized colloidal sol. See,for example, U.S. Pat. No. 4,562,021 which teaches a method ofmanufacturing a medium for microfiltration, for ultrafiltration, or forreverse osmosis in which a sol of particles of an oxide or a hydroxideof a chemical element is formed, a thickening agent is added to the sol,and the resulting sol is slipcast onto a support layer having poreswhich are larger than the pores desired for the filter medium, the thinlayer deposited on the support medium being dried and then heat treatedto eliminate the thickening agent and to sinter the particles of thedeposited thin layer. See also, U.S. Pat. No. 5,096,745 which teaches aprocess for preparing particulate or polymeric titania ceramic membraneswhich includes the steps of preparing a colloidal solution containing atitanium organic salt with a specific ratio between water and titaniumconcentration in the colloid so as to determine whether the resultingmembrane is either particulate or polymeric, adding to the colloidalsolution an alkyl alcohol, and sintering the gel created from thecolloid into a ceramic so as to prevent cracking of the resultingmembrane. See also, U.S. Pat. No. 5,169,576.

Particulate sols consist generally of highly condensed ceramic particlesin the 2-200 nanometer size range obtained in SiO₂, Al₂ O₃, and TiO₂systems. In membranes prepared from monosized particulate sols obtainedby hydrolysis of metal salts or alkoxides, pore volume depends simply onthe particle packing, and pore size decreases linearly with the particlesize when aggregation is avoided. An advantage of the particulateapproach is that the porosity of the membrane is independent of poresize. However, the particulate approach has several disadvantages. Inparticular, colloidal stability is essential to avoid aggregation of theconcentrating particles which otherwise would result in a bimodal poresize distribution, that is, pores within and between aggregates. Inaddition, the small particles necessary to obtain small pore sizes haveassociated with them a relatively thick tightly bound solvent layer thatdecreases the volume fraction of solids in the deposited film. Theremoval of this solvent during drying creates tensile stresses withinthe plane of the film that results in cracking.

The polymeric approach for preparing inorganic membranes involvesslip-casting and calcination of a polymeric sol. Polymeric sols consistgenerally of more or less branched clusters that do not contain a fullycondensed ceramic core and are obtained in the SiO₂, Al₂ O₃, ZrO₂ andTiO₂ systems under conditions where the reaction rate is minimized; forthe case of non-silicates, complexation chemistry is often used toreduce the polymer functionality. The inorganic polymer approach formaking membranes offers several advantages. First, crack free membranelayers can be prepared where aggregation of polymeric precursors isexploited to control the pore size. Secondly, the size of the polymericspecies can be controlled so that the deposited membrane forms a thinlayer that spans the support with minimal pore plugging. And, finally,the physical and chemical characteristics of the membrane can bealtered, either in the sol stage or in the deposited membrane stage, toalter the surface chemistry, while maintaining control of the pore size.

A potential disadvantage of the polymeric approach is that small poresizes and narrow pore size distributions are achieved at the expense ofpore volume. As a result, the permeability of the membrane may decreaseto the point where the membranes are no longer practically viable.

U.S. Pat. No. 4,973,435 teaches a method for producing porous membranesof sinterable refractory metal oxides wherein a powder of the metaloxide is dispersed in an organic polymer in an amount such that, afterthe polymer has been carbonized in a subsequent step, there is astoichiometrical excess of the oxide to carbon. The solution is thenshaped to form a desired thin membrane and the polymer is thencarbonized by heating it in a non-oxidizing atmosphere. The resultingproduct is heated to a temperature at which the carbon reacts with theoxide to form a volatile sub-oxide and carbon monoxide and the remainingoxide particles sinter together. A method of manufacturing poroussintered inorganic bodies with large open pore volumes in which asinterable material in the form of finely ground powder is mixed with aleachable substance in the form of a powder, and a mixture of sinterablematerial and leachable substance is heated to a sintering temperatureand maintained there until the sinterable mass is sintered, after whichthe mass is then cooled and the leachable substance leached from thesintered mass is taught by U.S. Pat. No. 4,588,540. U.S. Pat. No.4,221,748 teaches a method for making porous, crushable cores having aporous integral outer barrier layer with a density gradient therein. Themethod includes the process steps of preparing a material compositionconsisting essentially of an organic binder, a reactant fugitive fillermaterial, and an alumina flour. A portion of the material composition isthen worked into a preform of a predetermined shape of the ceramicarticle to be produced. The preform is then heated to remove the organicbinder while retaining substantially all of the reactant fugitive fillermaterial therein. Heating is then continued in a controlled atmosphereto react the alumina and the reactant fugitive filler material toproduce at least one or more suboxides of alumina. The one or moresuboxides of alumina are vapor transported throughout the fired preformto produce a ceramic article having a predetermined porosity content,grain morphology, and crushability characteristics. A portion of thesuboxides of alumina are oxidized to form a porous integral barrierlayer of alumina at the surface of the ceramic article, the layer havinga density gradient across its thickness. The remainder of the suboxidesescape from the core resulting in a net weight loss. U.S. Pat. No.3,963,504 teaches a porous ceramic monolithic structure prepared byshaping a ceramic filled polyolefinic material containing a plasticizer,shaping, extracting the plasticizer and treating to remove thepolyolefin. Finally, U.S. Pat. No. 5,087,277 teaches a high temperatureceramic filter produced from a composition containing refractory cement,aggregate, pore forming additives and sintering agents.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a process for producingmolecular sieving silica membranes which overcomes the potentialdisadvantages of traditional approaches to producing such membranes asdiscussed hereinabove.

It is an object of this invention to provide a process for producing gasseparation membranes having high selectivity achieved by having smallpore sizes and narrow pore size distributions, and high permeance,defined as flux/pressure drop, achieved by having a large volumefraction porosity and a very thin selective layer.

It is yet another object of this invention to provide a process forproducing molecular sieving silica membranes using fugitive organicligands as micropore templates. In this approach, organic templates areintroduced in a dense inorganic matrix and then removed oxidatively orhydrolytically to create a microporous channel system that exhibitsmolecular sieving and/or molecular recognition characteristics. Ideally,the organic ligand volume fraction is used to control porosity enhancedflux, independently of selectivity, which depends on the ligand size andshape. In order to successfully implement this approach, severalcriteria must be satisfied. The organic ligands must be uniformlyincorporated in the inorganic matrix without aggregation or phaseseparation to avoid creating pores larger than the size of theindividual ligands; the synthesis and processing conditions shouldresult in a dense embedding matrix so that pores are created only bytemplate removal; and template removal should be achieved withoutcollapse of the matrix, so that the pores created preserve the originalsize and shape of the template.

These and other objects of this invention are achieved by a process forproducing a molecular sieve silica membrane comprising depositing ahybrid organic-inorganic polymer comprising at least one organicconstituent and at least one inorganic constituent on a porous substratematerial and removing at least a portion of the organic constituent ofthe hybrid organic-inorganic polymer, forming a porous film. Moreparticularly, the process for producing a molecular sieve silicamembrane in accordance with one embodiment of this invention comprisesgenerating a sol comprising a precursor of a hybrid organic-inorganicpolymer, storing the sol in a quiescent state under conditions suitablefor promoting hydrolysis, condensation, and/or ripening, forming ahybrid organic-inorganic polymer, coating a substrate material with thehybrid organic-inorganic polymer, and removing at least a portion of anorganic constituent of the coating, forming a porous film.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of this invention will be betterunderstood from the following detailed description taken in conjunctionwith the drawings wherein:

FIG. 1 is a diagram of the organic template approach of this inventionshowing the progressive densification of the inorganic matrix andcreation of micropores by template removal during calcination;

FIG. 2 is a cross-sectional TEM image of a 550° C. calcined 10 mol %MTES/TEOS membrane layer deposited on a U.S. Filter γ-Al₂ O₃ membranesupport where region A indicates the γ-Al₂ O₃ support and region B isthe external microporous SiO₂ layer;

FIG. 3 is a graphical representation of the refractive index and volumepercent porosity of MTES/TEOS films as a function of calcinationtemperature;

FIGS. 4A and 4B are a graphical representations of N₂ sorption isothermsof a 10 mol % MTES/TEOS xerogel as a function of calcinationtemperature;

FIG. 5 is a graphical representation showing single gas permeance dataof 10 mol % MTES/TEOS membranes as a function of calcinationtemperature;

FIG. 6 is a graphical representation showing a comparison CO₂ and CH₄permeances of 10 mol % and 40 mol % MTES/TEOS membranes as a function ofcalcination temperature;

FIG. 7 is a graphical representation showing a comparison of CO₂permeance and CO₂ /CH₄ separation factor of known organic membranes andthose of microporous inorganic membranes prepared in accordance with theprocess of this invention;

FIG. 8 shows the relative hydrophobicity of films prepared in accordancewith the process of this invention after (a) drying at 150° C., (b)calcining in an oxidizing atmosphere at 550° C., and (c) calcining in areducing atmosphere at 550° C.;

FIG. 9 shows the FTIR spectra of (a) A2 films calcined in air at 400° C.and (b) 55 mol % MTES/TEOS films calcined in argon at 550° C.; and

FIG. 10 shows a comparison of N₂ at 77K sorption isotherms of 55 mol %MTES/TEOS xerogel pyrolyzed under (a) oxidizing atmosphere at 550°C./0.5 hours, (b) reducing atmosphere at 550° C./0.5 hours, and (c)reducing atmosphere at 800° C./0.5 hours.

DESCRIPTION OF PREFERRED EMBODIMENTS

The organic template approach to producing molecular sieving silicamembranes in accordance with the process of this invention comprises solor gel preparation, sol aging, sol dilution and membrane deposition. Thefirst step of the process of this invention is sol or gel preparation.Suitable sols for use in accordance with the process of this inventioninclude polymeric sols (see Brinker and Scherer, Sol-Gel Science,Academic Press, San Diego, 1990, pages 6 and 7). Sol compositionsinclude partially or fully hydrolyzed metal alkoxides (M(OR)_(n)), whereM is a metal, R is an organic ligand, preferably an alkyl group, and nis an integer having a value equal to a coordination number of saidmetal. Also suitable are partially or fully hydrolyzed organicallysubstituted metal alkoxides (R'_(x) M(OR)_(n-x)), where R is an organicligand, R' is a non-hydrolyzable organic ligand that includes alkyl,aryl, or other polymerizable organic ligands, M is a metal, and n and xare integers, where n has a value equal to a coordination number of saidmetal and x is less than n, and partially or fully hydrolyzedorganically modified bridged metal alkoxides (M(OR)_(n-1)--R"--M(OR)_(n-1)), where R is an organic ligand, R" is anon-hydrolyzable rigid or flexible organic ligand that includes alkyl,alkylene, aryl, or other polymerizable organic ligands, M is a metal,and n is an integer having a value equal to a coordination number ofsaid metal.

Suitable fluid components of the sols in accordance with the process ofthis invention include alcohol or other organic fluids such as hexane,cyclohexane, toluene, tetrahydrofuran, acetonitrile, methoxyethanol, ormulticomponent, preferably miscible, fluid mixtures. The only practicalrequirements of sols suitable for use in the process of this inventionare that they remain stable, that is they do not phase separate orprecipitate, during the coating operation, and that the viscosity andconcentration are appropriate for the pertinent coating operation.

Sol aging refers to storage of the sol normally in a quiescent stateunder conditions appropriate to further the extents of hydrolysis,condensation, and/or ripening (see Brinker and Scherer, Sol-Gel Science,ibid, Chapter 6). In accordance with the process of this invention,aging is used prior to gelation to grow polymeric species such thatpolymers are captured on top of the support with minimum porepenetration. Sol aging is also used in organically modified metalalkoxide and composite systems to uniformly incorporate organic ligandswithout aggregation of the organic and inorganic phases. For the solclusters characterized by a mass fractal dimension, sol aging can beoptimized to make the polymer species mutually transparent, so that theywill interpenetrate during deposition and drying. Thus, aging can beused to promote collapse of the network during deposition and drying,thereby creating very small pores and very narrow pore sizedistributions in gels and membranes. In accordance with one preferredembodiment of the process of this invention, aging is carried out attemperatures in the range of about 25-90° C. and at protonconcentrations of about 10⁻¹ to about 10⁻⁵ M.

Film/membrane deposition in accordance with the process of thisinvention may be carried out by any suitable operation known to thoseskilled in the art, such as dip-coating or drainage, spin-coating, orother liquid-to-solid coating operations. The coating may be applied toany suitable support including, but not limited to, dense siliconwafers, glass slides, porous supports with a wide range of pore sizesand porosities, microporous glass fibers, and porous ceramic modules.During the coating operation, the polymer clusters are concentrated byevaporation of the fluid component of the sol, leading to the creationof a physical or chemical gel. The gel network is subjected to acapillary pressure described by the Kelvin equation, the magnitude ofwhich depends primarily on the composition of the pore fluid, pore size,and the relative pressure of the pore fluid constituents in theoverlying gas. In accordance with a particularly preferred embodiment ofthe process of this invention, shrinkage of the gel network in responseto capillary stresses is maximized without 4separation of the inorganicand organic phases such that any pores created in the matrix are ofmolecular dimensions.

In accordance with one preferred embodiment of the process of thisinvention, the as-deposited films are subjected to thermal treatment tocomplete the drying process, partially consolidate the film throughcontinued condensation reactions, partially or fully consolidate thefilm through sintering, and pyrolyze the residual organic ligands.Pyrolysis of the organic ligands increases the film/membrane porosity.Under these conditions, the size and shape of the organic ligand can beused to create pores with precisely controlled size, shape andthree-dimensional topologies. The volume fraction of the organic ligandsmay be used to control the volume fraction of the porosity of the film.

In accordance with another preferred embodiment of the process of thisinvention, chemical treatments, such as ozonolysis, oxygen plasma,photolysis, and selective dissolution can be used to remove residualorganic constituents in order to confer additional porosity to the film.It will be apparent to those skilled in the art that more than oneorganic ligand may be utilized in the synthesis to arrive at a compositestructure in which some organic ligands are removed to create porositywhile others are retained to provide hydrophobicity.

In accordance with yet another preferred embodiment of the process ofthis invention, the deposited films are subjected to surfacederivatization. Surface derivatization refers to themonolayer-by-monolayer reduction of the pore size and/or alteration ofthe pore surface chemistry by reaction of reactive terminal sitesexisting on the surface of the pores with molecules, oligomers, orpolymers. Derivatization agents suitable for use in accordance with theprocess of this invention include, but are not limited to,organofunctional silanes, such as chlorosilanes (R'_(x) SiCl_(4-x))where R' is an alkyl ligand and x is an integer having a value less than4; alkyl alkoxysilanes (R'_(x) Si(OR)_(4-x)), where R is an alkylligand, R' is a non-hydrolyzable ligand such as alkyl, fluoroalkyl, oramine, and x is an integer having a value less than 4; metal alkoxides,M(OR)_(n), where M is a metal selected from the group consisting ofsilicon, titanium or zirconium metal, R is an alkyl ligand and n is aninteger having a value equal to a coordination number of said metal; andalcohol amines, for example, triethanol amine; carboxylic acids, forexample, acetic acid, and β-diketonates, for example, acetylacetonate.

The collapse of the gel network under capillary stresses during dryingdictates the final pore size and volume fraction porosity of theas-deposited films and membranes. It is controllable by the extent ofcondensation of the polymer network, the extent of organic ligandloading, the magnitude of capillary pressure, the reaction conditionsthat favor uniform incorporation of the ligands without phaseseparation, and aging time. That is, reaction and film/membranedeposition conditions that increase the extent of capillary-stressinduced collapse of the gel network are preferred so that the collapseof the pore structure completely eliminates pores or creates pores ofmolecular dimensions. The final pore size, volume fraction porosity, andpore size distribution in films and membranes produced in accordancewith the process of this invention is established by the original porestructure in the as-deposited films and membranes and any porositycreated or lost during subsequent processing steps designed to removethe pore templates and further consolidate the inorganic matrix.

Aging time is used to grow the polymer clusters in the sol whilemaintaining the mutual transparency of the clusters, such that duringfilm/membrane deposition, the clusters can interpenetrate freely and, atthe same time, are captured on top of the support with minimum porepenetration. Under these conditions, control of the sol concentrationand coating rate enables the formation of thin layers, less than about500 nm, that consistently dry without cracking, leading to defect-freelayers.

The as-deposited films and membranes with ultramicropores may be furthersubjected to pyrolysis or oxygen plasma treatment to remove the templateligands. The microstructure created by template removal is controlled bythe size and shape of the organic ligands, the residual porosity of thematrix, network relaxation after template removal, aggregation oftemplates, pyrolysis temperature of the template ligands with respect toalkoxy ligands (that is, template removal before or after alkoxypyrolysis), and pyrolysis atmosphere. In accordance with a particularlypreferred embodiment of the process of this invention, the organictemplates are pyrolyzed under conditions that promote network relaxationand partial/complete sintering that removes the residual porosity of thematrix prior to template removal (See FIG. 1). Under these conditions,pores are created only by the removal of the organic template ligands.The reaction conditions that favor uniform dispersion of the organictemplates in the inorganic matrix without aggregation or phaseseparation are preferred in order to insure that the pores createdpreserve the original size and shape of the template.

The pore structure of the final film or membrane may be further modifiedby surface derivatization and subsequent heat treatment. Surfacederivatization can be performed using dilute solutions of monomers in avariety of solvents, for example Si(OR)₄ or R'Si(OR)₃, where R and R'equal CH₃, C₂ H₅, and/or C₆ H₅, to reduce the pore size, narrow the poresize distribution, heal defects, and/or alter the surface chemistry ofthe films and the membranes to impart specific adsorption/diffusioncharacteristics. In accordance with one embodiment of this invention,further heating may be employed to partially sinter the films ormembranes to further reduce the pore size and/or narrow the pore sizedistribution.

EXAMPLE 1

This example describes an optimized process for preparing molecularsieving silica membranes in accordance with one embodiment of thisinvention in which fugitive organic ligands serving as microporetemplates are pyrolyzed to create very small pores. Silicate sols wereprepared by co-polymerization of tetraethoxysilane (TEOS) andmethyltriethoxysilane (MTES) dissolved in ethanol using a two-step acidcatalyzed procedure. In the first step, MTES, TEOS, EtOH, H₂ O, and 1MHCl with molar ratios x:1-x:3.8:1.1:7.0×10⁻⁴, where x ranged from 10 to55 mol %, were refluxed at 60° C. for 90 minutes with stirring at 200rpm. In the second step, additional water and 1M HCl were added at roomtemperature, resulting in the final molar ratio of x:1-x:3.8:5.1:0.056.Typically, 30 ml of this sol was filtered using 0.2 microns PTFE syringefilters in a 125 ml Nalgene container and allowed to age at 50° C. Thegelation time of the sols depended on the MTES content, varying fromapproximately 50 hours for 10 mol % MTES/90 mol % TEOS sol toapproximately 75 hours for a 55 mol % MTES/45 mol % TEOS sol. The solswere typically aged for a t/t_(gel) =0.25-0.50 and diluted 1:2 withethanol that was filtered, also using a 0.2 micron filter (volumesol:volume EtOH) to obtain a sol suitable for coating. The volume ofethanol and the coating speed dictated the final thickness of the filmscoated on silicon wafers, whereas the size of the polymer clusters inthe sol, along with the above mentioned factors, dictated the finalthickness of external membrane layers coated on porous supports. Poroussupports were cleaned using a CO₂ SNOGUN™ cleaner and preheated to thesubsequent heat treatment temperature of the membrane, and outgassed at150° C. for 6 hours on under flowing UHP N₂ prior to membranedeposition. Following the above procedure, we were able to prepare filmson silicon wafers in the thickness range of 160-260 nm, and membranes onporous tubular supports in the thickness range of 40-125 nm on top ofthe support with some sol penetration into the pores of the support asshown in FIG. 2. In addition, the cross-sectional TEM micrograph in FIG.2 shows the 10 mol % MTES/TEOS membrane layer deposited on the γ-Al₂ O₃tubular support to be featureless and crack free under the depositionconditions. The as-deposited films exhibited a porosity in the 10-15%range depending on the MTES content as shown in FIG. 3 and the membranesshowed molecular sieving behavior (see columns 4, 5, 6 and 7 in Table 1hereinbelow).

                                      TABLE 1    __________________________________________________________________________             Time (hr) @             Temperature                   He             CO.sub.2    Membrane (° C.)                   Permeance                         .sup.α He/SF.sub.6                              .sup.α He/N.sub.2                                  Permeance                                        αCO.sub.2 /CH.sub.4    __________________________________________________________________________    10% MTES/TEOS             0.5 @ 150                   2.55 × 10.sup.-3                         12.1 8.7 2.29 × 10.sup.-3                                        1.5    t/tgel = 0.24             0.5 @ 400                   2.24 × 10.sup.-2                         7.2  1.3 1.81 × 10.sup.-2                                        1.2             4.0 @ 550                   2.31 × 10.sup.-3                         24.3 15.4                                  2.57 × 10.sup.-3                                        12.2    Surface  4.0 @ 400                   1.32 × 10.sup.-4                         328  14.4                                  2.04 × 10.sup.-4                                        71.5    Derivatization with    1:12 TEOS    monomer    40% MTES/TEOS             0.5 @ 150                   1.34 × 10.sup.-3                         12.0 2.2 1.72 × 10.sup.-3                                        1.8    t/tgel = 0.24             0.5 @ 400                   2.71 × 10.sup.-3                         15.2 2.2 3.29 × 10.sup.-3                                        2.0             4.0 @ 550                   4.64 × 10.sup.-3                         14.3 2.4 6.79 × 10.sup.-3                                        3.2    Surface  4.0 @ 400                    2.0 × 10.sup.-4                         47.6 7.7  5.0 × 10.sup.-4                                        36.5    Derivatization with    1:12 TEOS    monomer    __________________________________________________________________________

EXAMPLE 2

This example describes the enhancement of film porosity and membraneflux by a simple low-temperature heat treatment procedure usingmembranes prepared in accordance with Example 1. The films and membranesprepared from Example 1, having a volume percent porosity of about 14%,were placed in a quartz tube with flowing air and heated in a boxfurnace with a heating and cooling rate of 1° C. per hour. The filmsdip-coated on silicon wafers and membranes dip-coated on porous supportswere pyrolyzed at 400° C. for 0.5 hours. The membranes were furtheroutgassed at 150° C. for six hours in flowing ultrahigh purity (UHP)nitrogen. The porosity of the films after 400° C. heat treatmentincreased to about 15 to 20 volume percent depending on the MTEScontent, and the flux through the membranes increased by a factor of 5to 10 depending on the size of the gases studied. FIG. 5 illustratessingle gas permeance data for 10 mol % MTES/TEOS membranes.

The consequence of removing the organic templates from xerogel (driedgel) bulk samples prepared under identical conditions is illustrated inFIGS. 4A and 4B which show the N₂ sorption isotherm of the 10 mol %MTES/TEOS xerogels as a function of calcination temperature along with apartial CO₂ isotherm of the 550° C. sample. The N₂ sorption isothermsappear to change from Type I, characteristic of microporous materials at150° C. and 450° C., respectively, to Type II, characteristic ofnon-porous materials after calcination at 550° C. However, the partialCO₂ isotherm shows the 550° C. sample also to be microporous (see insetin FIG. 4). The apparent discrepancy between the N₂ and CO₂ data arisesbecause the pores in the xerogel are so small that the diffusion of N₂at 77K is severely kinetically limited compared to CO₂ at 273K. By 550°C., the scale of porosity is apparently quite small and thedensification of the inorganic matrix is virtually complete, based onessentially zero uptake of N₂ at 77K versus CO₂ at 273K.

The consequence of removing the organic templates from membranesprepared in accordance Examples 1 and 2 is shown in FIG. 5 which showsthe single gas permeance data of 10 mol % MTES/TEOS membranes as afunction of calcination temperature. After pyrolysis at 550° C. for 4hours to remove the methyl ligands, and outgassing at 400° C. for 6hours under UHP N₂, the permeance of larger gas molecules such as N₂,CH₄, and SF₆, decrease dramatically compared to He and CO₂. Theseparation factor for the gas pairs, CO₂ /CH₄, He/N₂, He/SF₆ (12.2,15.4, 24.3, respectively) are well above the ideal Knudsen values andincrease as the difference in their kinetic diameters increases,implying that a molecular sieving mechanism governs transport (see FIG.5 and columns 4, 5, and 7 of Table 1). The CO₂ and He permeance of themembranes after the 550° C. heat treatment were in general higher thanthose measured after drying at 150° C., indicating a net creation ofporosity by this approach.

EXAMPLE 3

This example describes the control of the porosity of films and thepermeance of membranes by varying the mol % loading of the organicligands. The carbon dioxide and methane permeances of membranes preparedfrom 10 mol % MTES and 40 mol % MTES are shown in FIG. 6. After a 550°C. heat treatment to pyrolyze the ethoxy and methyl ligands, the 40 mol% MTES membranes exhibited carbon dioxide and methane permeances thatwere about 2 to 10 times greater than the corresponding fluxes of 10 mol% MTES membranes. This shows that permeance is at least partiallycontrolled by the volume fraction of the template addition. Similarly,the porosity of the 40 mol % MTES films after 400° C. pyrolysis wereapproximately twice that of 10 mol % MTES films.

EXAMPLE 4

This example describes the surface derivatization of the pore surfacesof the membranes prepared in accordance with Examples 2 and 3, using avery dilute solution of TEOS. The membranes described above weredip-coated in a sol containing monomeric TEOS that was diluted 1:12 withethanol (volume TEOS:volume ethanol) and calcined at 400° C. for 4 hoursand outgassed at 400° C. for 6 hours under flowing UHP N₂. After twosurface derivatization treatments, the He/SF₆, He/N₂, and CO₂ /CH₄separation factors were 328, 14.4, 71.5, respectively, for the 10 mol %MTES/TEOS membranes. The corresponding separation factors for the 40 mol% MTES/TEOS membranes were 47.6, 7.7, and 36.5, respectively. This largeincrease in separation factor with only a modest reduction in CO₂ and Hepermeance is attributed to monolayer-by-monolayer reduction in the poresize of the membranes. The efficacy of the surface derivatizationapproach in accordance with one embodiment of this invention is apparentwhen we compare the results for MTES/TEOS membranes with known organicpolymer membranes as shown, for example, in FIG. 7. Compared to organicpolymer membranes exhibiting separation factors in the range of 70-80,the microporous inorganic membranes exhibit more than 1000 times greaterpermeance.

The kinetics of stress development and relaxation of thin films exposedto a series of alcohol molecules with increasing molecular diameters wasused as a molecular probe technique to determine the influence oftemplate ligand size and shape on the resulting pore size. For TEOS, 25mol % MTES/TEOS and 25 mol % PTMS (phenyltrimethoxysilane/TEOS) filmsprepared under identical conditions and pyrolyzed at 550° C. under anoxidizing atmosphere, the radius of the largest alcohol molecule thatfit into the pores increased from 0.38 nanometers (iso-propanol) to 0.41nanometers (t-butanol) to 0.45 nanometers (3, 5, dimethylbenzylalcohol), respectively. These data show that pore radius increaseswith template size for methyl and phenyl templated silicas, but theaverage pore radius somewhat exceeds the templates sizes (estimated asapproximately 0.19 nanometers for methyl and approximately 0.34nanometers for phenyl).

With regard to the correspondence between volume fraction template andvolume fraction porosity, it is generally observed that the pore volumefraction is less than that of the templates for molar percentages oforganotrialkoxysilanes exceeding about 10%. This stems from relaxationof the network that accompanies template pyrolysis and any enhancedsintering. This effect is more pronounced in systems containing largetemplate concentrations and, especially, for low pyrolysis temperaturesdue to the lower extents of condensation of the matrix at the moment thetemplates are pyrolyzed for these situations. These effects can beminimized by increasing the extent of condensation of the matrix priorto template pyrolysis.

For practical applications, molecular sieving membranes must exhibitlong term thermal and chemical stability. Thermal stability refers tothe stability towards densification of the silica matrix at hightemperatures by mechanisms such as structural relaxation and viscoussintering. Due to their small pore sizes, bulk microporous xerogelsdensify by 550° C., resulting in significant loss of surface area andporosity. However, as previously stated, the constraint imposed by thesupport prevents significant densification of thin films and membranesin the 200 to 600° C. temperature range.

Another consequence of reducing the pore size in amorphous silicamembranes is that microporous silica is known to undergo aging in wateror steam environments, often resulting in a loss of surface area andpore volume. For membranes, this can result in a reduction in flux andpossibly separation factor. Due to the extremely small pore sizes, thedriving force for water adsorption is quite high. Even very low waterlevels can cause long term stability problems, resulting indeterioration of the membrane performance.

One approach for dealing with the problem of water condensation in thepores of molecular sieving membranes is to make the pore surfacehydrophobic. This is achieved with membranes produced in accordance withone embodiment of the process of this invention by heating the membranesunder reducing conditions. FIG. 8 compares the advancing water contactangles, θ, for these films after (a) drying at 150° C., (b) calcining inan oxidizing atmosphere (air) at 550° C. and, (c) calcining in areducing atmosphere at 550° C. In accordance with a particularlypreferred embodiment of the process of this invention, said reducingatmosphere comprises a gas selected from the group consisting of argon,nitrogen, 4% hydrogen/96% nitrogen, and mixtures thereof. The watercontact angles, that is the hydrophobicity, of the as-dried filmsincreased from 45 to 80° with increases in the MTES/TEOS mol ratio from0 to 55%. Calcination in air to pyrolyze the methyl ligands decreasedthe water contact angles, that is decreased the film hydrophobicity, toabout 35°, regardless of the MTES concentration. However, calcinationunder reducing atmospheric conditions increased the water contact angleθ to greater than 100°. The latter values increased with increasingmethyl ligand content and remained the same even after storing the filmsat 150° C. and approximately 50% relative humidity for three months. Forexample, increasing the MTES/TEOS mol ratios from 0 to 55 mol %increased the water contact angle θ from 80° to 104°. These resultsclearly establish that the long term hydrolytic stability of molecularsieving membranes produced in accordance with the process of thisinvention can be enhanced by calcination in a reducing atmosphere.

FIG. 9 shows the FTIR spectra of (a) A2 films calcined in air at 400° C.and (b) 55 mol % MTES/TEOS films calcined in argon at 550° C. Thespectra are truncated to clarify the water absorption in the two films.The common features in both the spectra are the presence of Si--O--Sipeak (1090 cm⁻¹) and the Si--O--Si shoulder (1221 cm⁻¹). The 1221 cm⁻¹peak is more pronounced in the hydrophobic films due to the overlap ofSi--O--Si stretch with the Si--O--Si stretch of silicons attached tocarbons (1250-1260 cm⁻¹). The significant differences between the twospectra are the presence of symmetric --CH₃ stretch (2960 cm⁻¹), C--H(1340 cm⁻¹), and the lack of any absorption in the SiO--H region(3200-3700 cm⁻¹) for the hydrophobic film. The results show that thefilms calcined under reducing conditions have a hydrophobic surfaceterminated by SiC bonds and are consistent with the water contact angleθ of 104° measured for these films.

FIG. 10 shows a comparison of nitrogen at 77K sorption isotherms of 55mol % MTES/TEOS xerogel pyrolyzed under (a) an oxidizing atmosphere at550° C./0.5 hours, (b) a reducing atmosphere at 550° C./0.5 hours, and(c) a reducing atmosphere at 800° C./0.5 hours. The xerogels calcinedunder oxidizing conditions are hydrophilic with a water contact angle ofapproximately 30 to 40° based upon measurements of corresponding films.By comparison, the xerogels calcined under reducing conditions arehydrophobic with a water contact angle θ of about 102 to 106° based uponmeasurement of corresponding films.

As shown in FIG. 10, the hydrophilic xerogels are almost non-porous tonitrogen (Type II isotherm) after calcination at 550° C., whereas thehydrophobic xerogels exhibit a Type IV isotherm characteristic ofmesoporous materials. In addition, the microstructure of the hydrophobicxerogels remains nearly the same on heating to 800° C. in a reducingatmosphere. This is due to the incorporation of carbon in the network assilicon carbide or silicon oxycarbide that impart refractory-likecharacter to the network. The high viscosity of the network due toincorporation of carbon combined with low surface free energies (lessthan about 30 dyne/cm, measured from contact angle experiments) shiftsthe onset of viscous sintering to higher temperatures. The larger poresize of the hydrophobic xerogels is due to redistribution reactions thatremove highly volatile small cyclic and/or oligomeric species around450-550° C. However, as shown above, much smaller pore sizes can beachieved in thin films through proper choice of sol composition, agingtime, and deposition conditions.

These high surface area (600 m² /g) hydrophobic xerogels, thin films,and membranes have applications in catalysis (as catalyst supports),water repellant coatings, thermally and hydrolytically stable molecularsieving membranes, and sensors.

While in the foregoing specification this invention has been describedin relation to certain preferred embodiments thereof, and many detailshave been set forth for purpose of illustration, it will be apparent tothose skilled in the art that the invention is susceptible to additionalembodiments and that certain of the details described herein can bevaried considerably without departing from the basic principles of theinvention.

We claim:
 1. A process for producing a molecular sieve silica membranecomprising:depositing a hybrid organic-inorganic polymer comprising atleast one organic constituent and at least one inorganic constituent ona porous substrate material, said hybrid organic-inorganic polymercomprising at least one compound selected from the group consisting ofpartially hydrolyzed alkoxides, fully hydrolyzed alkoxides, partiallyorganically substituted metal alkoxides, partially hydrolyzedorganically modified metal alkoxides, fully hydrolyzed organicallymodified metal alkoxides, partially hydrolyzed organically modifiedbridged metal alkoxides, fully hydrolyzed organically modified bridgedmetal alkoxides, and mixtures thereof; and removing at least a portionof said at least one organic constituent of said hybridorganic-inorganic polymer, forming a porous film.
 2. A process inaccordance with claim 1, wherein said hybrid organic-inorganic polymercomprises at least one of said partially hydrolyzed alkoxides and saidfully hydrolyzed alkoxides having the formula

    M(OR).sub.n

where M is a metal, R is an organic ligand and n is an integer having avalue equal to a coordination number of said metal.
 3. A process inaccordance with claim 2, wherein said organic ligand is an alkyl group.4. A process in accordance with claim 2, wherein said metal is selectedfrom the group consisting of silicon, titanium and zirconium.
 5. Aprocess in accordance with claim 1, wherein said hybridorganic-inorganic polymer comprises said partially organicallysubstituted metal alkoxides having the formula

    R'.sub.x M(OR).sub.n-x

where R is an organic ligand, R' is a non-hydrolyzable organic ligand, Mis a metal, n and x are integers, n having a value equal to acoordination value of said metal, and x<n.
 6. A process in accordancewith claim 5, wherein said non-hydrolyzable organic ligand is apolymerizable organic ligand selected from the group consisting of allylligands, acryl ligands, vinyl ligands, epoxy ligands and mixturesthereof.
 7. A process in accordance with claim 5, wherein said metal isselected from the group consisting of silicon, titanium and zirconium.8. A process in accordance with claim 1, wherein said hybridorganic-inorganic polymer comprises at least one of said partiallyhydrolyzed organically modified bridged metal alkoxides and said fullyhydrolyzed organically modified bridged metal alkoxides having theformula

    M(OR).sub.n-1 --R"--M(OR).sub.n-1

where R is an organic ligand, R" is a non-hydrolyzable rigid or flexibleorganic ligand, M is a metal, and n is an integer having a value equalto a coordination number of said metal.
 9. A process in accordance withclaim 8, wherein said non-hydrolyzable organic ligand is a polymerizableorganic ligand selected from the group consisting of allyl ligands,acryl ligands, vinyl ligands, epoxy ligands and mixtures thereof.
 10. Aprocess in accordance with claim 8, wherein said metal is selected fromthe group consisting of silicon, titanium and zirconium.
 11. A processin accordance with claim 1, wherein said hybrid organic-inorganicpolymer is prepared from a sol using a sol-gel process.
 12. A process inaccordance with claim 11, wherein said sol comprises an organic fluidselected from the group consisting of an alcohol, hexane, cyclohexane,toluene, tetrahydrofuran, acetonitrile, methoxyethanol, and mixturesthereof.